Welded tube with excellent corrosion-resistant inner surface and method of producing the same

ABSTRACT

Disclosed is a welded tube with an excellent corrosion-resistant inner surface, in which the inner surface including the bead part has a first plate layer of one metal material selected from Sn, Sn--Zn, Sn--Ni, Ni--P and Ni--B and the first plate layer is overcoated with a second plate layer of one metal material selected from Ni, Co and alloys based on the metals. The welded tube is free from exposure of the steel base out of the plate layer and is also free from cracks, pin holes, overplated spots and peeling of the plate layer. It has excellent corrosion resistance and good workability for working the terminals. The welded tube is produced by forming a first plate layer of Sn, Sn--Zn, Sn--Ni, Ni--P or Ni--B on at least one surface of a steel strip, then forming a second plate layer of Ni, Co or an alloy based on the metals over the first plate layer, shaping the thus plated steel strip into a tube by welding with the plated surface being inside and then heat-treating the tube.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a welded with an excellentcorrosion-resistant inner surface, in which the inner surface as platedwith a metal or alloy has excellent corrosion resistance to be free fromcorrosion especially that from the welded part (hereinafter referred toas a bead part) and also to a method of producing the same.

2. Description of the Prior Art

Heretofore, a small-diameter steel tube has been used as a fuel pipe forcars. As pipes for lower gasoline, sour gasoline, alcohol andalcohol-mixed fuels, formation of an anti-rusting metal layer on theinner surface thereof for improving the rust resistance of it has beenproposed. For instance, there has been used a welded steel tube withimproved inner surface rusting resistance to be produced by a method inwhich at least one surface of a steel strip is plated with nickel byelectric plating, the thus plated strip is formed into a tube by rollforming with the plated inner surface being inside, the butt joint areaof the rolled tube is welded by electric resistance welding withelectrode rolls or the like and the tube is finally annealed.

However, since the welded tube of the kind having a nickel plate layeron the inner surface thereof is produced by welding the butt joint areathereof by electric resistance welding or the like after the rollforming, the bead part to be formed on the inner surface of the tubeoften has discontinuous areas also in the nickel plate layer thereon tocause local exposure of the steel base out of the plate layer (see FIG.2), or as the case may be, pin holes existing in the nickel plate layerin the part would often remain as they are. As a result, the partinvolves a problem that it would easily be corroded by water to be inalcohol or alcohol-mixed fuel or by organic acids to be formed byoxidation of gasoline or decomposition of alcohol.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the above-mentionedproblem and to provide a welded tube in which the bead part thereof iswell plated with a plate layer and the pin holes are also well coatedwith the layer to have not only excellent corrosion resistance but alsoexcellent workability, for example, for working terminals of the tube,and also to provide a method of producing the welded tube.

The present inventors repeatedly studied so as to overcome theabove-mentioned problem and to attain the above-mentioned object andhave found that the object may be attained when a steel strip as coatedwith a plate layer of a low melting point single metal or alloy, such asSn, Sn--Zn, Ni--P or the like, and then overcoated with another platelayer of at least one metal or metal alloy having a higher melting pointthan that of the metal or alloy of the previous plate layer, such as oneselected from Ni, Co and alloys based on the metals, over the previousplate layer is used and formed into a tube. In the basis of the finding,they have completed the present invention. Therefore, the firstembodiment of the present invention is a welded tube with an excellentcorrosion-resistant inner surface, in which the inner surface includingthe bead part has been coated with a first plate layer made of at leastone of Sn, Sn--Zn, Sn--Ni, Ni--P and Ni--B and the first plate layerpreferably including the bead part has been overcoated with a secondplate layer made of at least one of Ni, Co and alloys based on themetals. The second embodiment of the present invention is a method ofproducing a welded tube with an excellent corrosion-resistant innersurface, in which a first plate layer of at least one selected from Sn,Sn--Zn, Sn--Ni, Ni--P and Ni--B is formed on at least one surface of asteel strip, then a second plate layer of at least one selected from Ni,Co and alloys based on the metals is formed over the first plate layer,and the thus plated steel strip is formed into a tube with the platedsurface being inside and heat-treated.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an enlarged cross-sectional view of showing one embodiment ofa welded tube of the present invention, cut in the direction of thediameter of the tube.

FIG. 2 is an enlarged cross-sectional view of showing one example of aconventional welded tube, cut in the same manner as in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The steel strip for use in the present invention may be anyone which isgenerally used as a material for fuel pipes for cars.

The first plate layer of a low melting point material is formed byelectric or chemical plating of a single metal or alloy having a lowermelting point than that of metals or alloys of forming the second platelayer, which will be mentioned below, and selected from the groupconsisting of Sn, Sn--Zn, Sn--Ni, Ni--P and Ni--B, and it preferably hasa thickness of from 0.5 to 10 μm. If the thickness of the first platelayer is less than 0.5 μm, it would be difficult to fully compensate thefailure of the second plate layer, which will be mentioned below, suchas pin holes or cracks of the same and to fully cover the exposed areaof the steel base due to the discontinuous plate cover in the bead areaso that the corrosion resistance of the inner surface of the welded tubewould be insufficient. On the contrary, if it is more than 10 μm, theplate layer would be cracked or peeled during the post-working such asbending or pressing of the welded tube, but rather the effect of thepresent invention would not be augmented so much despite of such thickplating.

The second plate layer is formed over the first plate layer byconventional electric or chemical steel plating to have a thickness offrom 0.5 to 10 μm, using one selected from Ni, Co and alloys of them. Ifthe thickness of the second plate layer is less than 0.5 μm, theintended corrosion resistance could not be attained. On the contrary, ifit is more than 10 μm, the plate layer would be cracked and many stepsare needed for the plating to be expensive with giving no furtheradditional advantage.

The total thickness of the first layer and the second layer is desiredto fall within the range of from 1 to 15 μm. If it is less than 1 μm,the corrosion resistance would be insufficient. On the contrary, if itis more than 15 μm, the plate layer would be cracked or peeled duringthe post-working such as bending or pressing of the welded tube.

Formation of the thus plated steel strip into a welded tube may beeffected by any conventional process, for example, comprising cuttingthe strip into a desired size, roll-forming it into a tube with theplated surface being inside, welding the butt joint area to a weldedtube and heat-treating the tube.

The last heat treatment is effected in order that the first plate layeris fused to be fluid to thereby compensate the failure of the secondplate layer and also cover the bead part therewith with additionallyremoving the thermal influence of the previous welding on the weldedtube. Therefore, it is preferably effected at a temperature fallingwithin the range between the melting point of the single metal or alloyof the first plate layer and 1200° C. for a period of from 5 seconds to15 minutes. If the temperature is lower than the melting point, thefirst plate layer could not be fused to be fluid so that the failure ofthe second plate layer could not well be compensated and the bead partcould not well be covered therewith. On the contrary, if it is higherthan 1200° C., the steel base material would be heat-deteriorated due tothe growth of the crystal grains to coarse grains. More preferably, theheat treatment is effected at 600° C., since the thermal influence bywelding may well be removed and the interdiffusion between the firstplate layer and the second plate layer may further be promoted. If thetime for the heat treatment is less than 5 seconds, fusion andfluidization of the first plate layer would be insufficient so that thefailure of the second plate layer could not well be compensated and thebead part could not well be covered therewith. In addition, removal ofthe thermal influence by welding would be insufficient. On the contrary,if it is more than 15 minutes, the steel base material would beheat-deteriorated due to the growth of the crystal grains to coarsegrains and additionally, the producibility would lower. More preferably,the heat treatment is effected at a temperature falling within the rangeof from 800° to 1200° C. for a period of from 10 seconds to 5 minutes.

Specifically, in accordance with the present invention, at least onesurface of a steel strip is plated with a first plate layer of Sn or thelike, having a determined thickness, by an ordinary plating method, thena second plate layer such as a nickel plate layer or the like, having adetermined thickness, is formed thereover, then the two-layer platedsteel strip is cut into a desired width and formed into a tube by anordinary roll-filming method with the plated surface being inside, thebut joint area of the tube is welded by an ordinary electric resistancewelding method or high frequency welding method to form a welded tube,and thereafter the welded tube is heat-treated by the use of anatmospheric furnace or a high frequency heating furnace to produce awelded tube having an excellent corrosion-resistant inner surface. Inthis way, the above-mentioned problem has been overcome by the presentinvention.

In general, in the process of producing the welded tube of the kind, thebead part to be formed in the butt joint area by electric resistancewelding or high frequency welding is often to involve local failure ofthe plate layer as formed thereon. In accordance with the method of thepresent invention, however, the inner first plate layer of Sn or thelike having a lower melting point than the material of the outer secondplate layer is fused to be fluid due to the heat treatment to beeffected after welding of the plated strip whereby the fused and fluidmaterial of the first plate layer is to spread around the bead part dueto the capillary phenomenon or the like to additionally form a lowmelting point plate layer of Sn or the like around the part. Inaddition, even though the second plate layer of Ni or the like has pinholes or cracks, the low melting point plate layer of the first platelayer is also fused to be fluid due to the heat treatment to well fillthem because of the same reason as above. Therefore, the inner surfaceof the welded tube of the present invention is coated with the firstplate layer of Sn or the like and is further overcoated with the secondplate layer of a metal or alloy having a higher melting point than themetal or alloy of the first plate layer, while the interface between thefirst plate layer and the second plate layer has a diffusion phase ofthe two layers. As a result, at least the bead part, pin holes andcracks are coated and filled with the fused low melting point metallayer. In this way, the inner surface of the welded tube of the presentinvention is free from exposure of the steel base out of the plate layerand is therefore resistant to lower gasoline, sour gasoline, alcohol,water to be contained in alcohol-mixed fuel and organic acids due to thebarrier protection by the plate layer well formed on the inner surfaceof the tube.

The present invention will be explained in more detail by way of thefollowing examples, which, however, are not intended to restrict thescope of the present invention.

EXAMPLE 1

As shown in FIG. 1, a first Ni--P plate layer 2 was formed on a steelstrip (JIS G 3141 SPCC) 1 by chemical plating to have a thickness of 3μm. Next, a second Ni plate layer 3 was formed over the Ni--P platelayer by electric plating, using a known Watt plating bath, to have athickness of 3 μm. The total thickness of the first plate layer and thesecond plate layer was thus 6 μm.

The thus two-layer plated steel strip was cut into an open width of atube to be formed and shaped into a tube by a known roll-forming method,and the butt joint part of the tube was welded by electric resistancewelding to form a welded tube having an outer diameter of 8 mm. This washeat-treated at 1000° C. for 15 seconds. The welded tube thus obtainedwas divided into two in the direction of the axis of the tube, and onetest piece thus cut was masked except the area to be tested and wassubjected to a salt spray test on the basis of JIS Z 2371. As a result,no red rust formed on the inner surface of the test piece of the tubeeven after 96 hours. Thus, the tube was admitted to have excellentcorrosion resistance. By microscopic observation, it was confirmed thatthe first plate layer 2 was formed also on the bead part 4 and that theinterface between the first plate layer 2 and the second plate layer 3had a diffusion phase 5. In addition, the welded tube sample wassubjected to a bending test of bending it to 180 degrees at a radius of20 mm with a grooved roll and to a press working test of pressing theterminals of it for evaluation of the workability thereof, whereupon theplate layer neither cracked nor peeled.

EXAMPLE 2

A first Sn plate layer was formed on a steel strip of the same kind asthat used in Example 1 by electric plating using a known sulfuric acidbath, to have a thickness of 3 μm; and thereafter a second Ni platelayer was formed thereon by the same electric plating as that in Example1 to have a thickness of 5 μm, the total thickness of the first platelayer and the second plate layer being 8 μm. The thus two-layer platedsteel strip was formed into a tube in the same manner as in Example 1and then heat-treated at 300° C. for 10 seconds to produce a weldedtube.

This was subjected to the same salt spray test as in Example 1,whereupon no rust formed even after 72 hours. By microscopic observationof the cross section of the bead part of the tube, it was confirmed thatthe first plate layer wholly covered the bead part and that theinterface between the first plate layer and the second plate layer had adiffusion phase of the two layers. By the same bending test andevaluation test as in Example 1, the plate layer neither cracked norpeeled.

EXAMPLE 3

A first Sn--Zn plate layer was formed on a steel strip of the same kindas that used in Example 1 by electric plating using a plating liquid ofSZ-240 (produced by Dipsole Co.) to have a thickness of 0.5 μm. Then, asecond Ni--Co plate layer was formed over the first plate layer byelectric plating using a plating bath comprising 260 g/liter ofNiCl₂.6H₂ O, 14 g/liter of CoCl₂.6H₂ O and 15 g/liter of H₃ BO₃, to havea thickness of 0.5 μm, the total thickness of the first plate layer andthe second plate layer being 1 μm. The thus two-layer plated steel stripwas formed into a tube in the same manner as in Example 1 and thenheat-treated at 1000° C. for 1 minute to produce a welded tube.

This was subjected to the same salt spray test as in Example 1,whereupon no rust formed even after 48 hours. By microscopic observationof the cross section of the bead part of the tube, it was confirmed thatthe first plate layer wholly covered the part. By the same bending testand evaluation test as in Example 1, the plate layer neither cracked norpeeled.

EXAMPLE 4

A first Sn--Ni plate layer was formed on a steel strip of the same kindas that used in Example 1 by electric plating using a plating bathcomprising 28 g/liter of SnCl₂.6H₂ O, 30 g/liter of NiCl₂.6H₂ O, 200g/liter of K₄ P₂ O₇, 20 g/liter of glycine and 5 ml/liter ofconcentrated aqueous ammonia, to have a thickness of 0.5 μm. Then, asecond Co plate layer was formed over the first plate layer by chemicalplating using a plating bath comprising 22 g/liter of CoCl₂.7H₂ O, 105g/liter of N₂ H₅ Cl₂ and 90 g/liter of C₄ H₄ O₆ Na₂.2H₂ O, to have athickness of 8 μm, the total thickness of the first layer and the secondlayer being 8.5 μm. The thus two-layer plated steel strip was formedinto a tube in the same manner as in Example 1 and then heat-treated at1130° C. for 30 seconds to produce a welded tube.

This was subjected to the same salt spray test as in Example 1,whereupon no rust formed even after 48 hours. By microscopic observationof the cross section of the bead part of the tube, it was confirmed thatthe first plate layer wholly covered the part. By the same bending testand evaluation test as in Example 1, the plate layer neither cracked norpeeled.

EXAMPLE 5

A first Ni plate layer was formed on a steel strip of the same kind asthat used in Example 1 by chemical plating to have a thickness of 5 μm,and a second Ni plate layer was formed thereover by the same electricplating as in Example 1 to have a thickness of 0.5 μm, the totalthickness of the first plate layer and the second plate layer being 5.5μm. The thus two-layer plated steel strip was formed into a tube in thesame manner as in Example 1 and then heat-treated at 1200° C. for 3minutes to produce a welded tube.

This was subjected to the same salt spray test as in Example 1,whereupon no rust formed even after 72 hours. By microscopic observationof the cross section of the bead part of the tube, it was confirmed thatthe first plate layer wholly covered the part and that the interfacebetween the first plate layer and the second plate layer had a diffusionphase of the two layers. By the same bending test and evaluation test asin Example 1, the plate layer neither cracked nor peeled.

EXAMPLE 6

A first Ni--P plate layer was formed on a steel strip of the same kindas that used in Example 1 by the same chemical plating as in Example 1to have a thickness of 5 μm, and then a second Ni--B plate layer wasformed thereover by the same chemical plating as in Example 3 for thefirst layer plating to have a thickness of 4 μm, the total thickness ofthe first plate layer and the second plate layer being 9 μm. The thustwo-layer plated steel strip was formed into a tube in the same manneras in Example 1 and then heat-treated at 900° C. for 30 seconds toproduce a welded tube.

This was subjected to the same salt spray test as in Example 1,whereupon no rust formed even after 72 hours. By microscopic observationof the cross section of the bead part of the tube, it was confirmed thatthe first plate layer wholly covered the part. By the same bending testand evaluation test as in Example 1, the plate layer neither cracked norpeeled.

COMPARATIVE EXAMPLE 1

Only an Ni plate layer of 3 μm thick, which is the same one as thesecond plate layer 2 of Example 1, was formed on a steel strip of thesame kind as that used in Example 1, as in FIG. 2. The thus plated stripwas formed into a tube in the same manner as in Example 1 and thenheat-treated at 900° C. for 30 minutes to produce a welded tube.

This was subjected to the same salt spray test as in Example 1,whereupon the bead part 4 rusted in 0.5 hour. By microscopic observationof the cross section of the bead part of the tube, it was confirmed thatthe Ni plate layer was not fully on the part.

COMPARATIVE EXAMPLE 2

A first Sn plate layer was formed on a steel strip of the same kind asthat used in Example 1 by the same electric plating as in Example 2 tohave a thickness of 0.2 μm, and a second Ni plate layer was formedthereover by the same electric plating as in Example 1 for the secondplating to have a thickness of 15 μm, the total thickness of the firstplate layer and the second plate layer being 15.2 μm. The thus two-layerplated strip was formed into a tube in the same manner as in Example 1and then heat-treated at 300° C. for one minute to produce a weldedtube.

This was subjected to the same salt spray test as in Example 1,whereupon no rust formed even after 24 hours. However, as a result ofthe bending test and evaluation test as applied to the tube, the platelayer peeled off. From this, it is understood that when the totalthickness of the first plate layer and the second plate layer to beformed on the inner surface of the welded tube is over the uppermostlimit of the range of from 1 μm to 15 μm, the tube is no morepracticable and is not economical because of the high cost of the rawmaterials to be used.

COMPARATIVE EXAMPLE 3

A first Ni--B plate layer was formed on a steel strip of the same kindas that used in Example 1 by the same chemical plating as that inExample 5 for the first plating to have a thickness of 15 μm, and then asecond Co--Sn plate layer was formed thereover by electric plating usinga plating bath comprising 260 g/liter of NICl₂.6H₂ O, 14 g/liter ofCoCl₂.6H₂ O and 15 g/liter of H₃ BO₃ to have a thickness of 5 μm, thetotal thickness of the first plate layer and the second plate layerbeing 20 μm. The thus two-layer plated strip was formed into a tube inthe same manner as in Example 1 and then heat-treated at 1200° C. for 5minutes to produce a welded tube.

This was subjected to the same salt spray test as in Example 1,whereupon no rust formed even after 24 hours. However, as a result ofthe bending test and evaluation test as applied to the tube, the platelayer peeled off.

COMPARATIVE EXAMPLE 4

A first Ni--P plate layer was formed on a steel strip of the same kindas that used in Example 1 by the same chemical plating as in Example 1for the first plating to have a thickness of 15 μm, and a second Niplate layer was formed thereover by the same electric plating as inExample 1 for the second plating to have a thickness of 15 μm, the totalthickness of the first plate layer and the second plate layer being 30μm. The thus two-layer plated strip was formed into a tube in the samemanner as in Example 1 and then heat-treated at 850° C. for 15 secondsto produce a welded tube.

This was subjected to the same salt spray test as in Example 1,whereupon no rust formed even after 4 hours. However, as a result of thebending test and evaluation test as applied to the tube, the plate layercracked and peeled off.

In accordance with the present invention as mentioned above in detail,the inner surface of a welded tube has two plate layers of a first platelayer and a second plate layer of a metal or alloy having a lowermelting point of the metal or alloy of the first plate layer.Accordingly, the welded tube of the present invention is free fromexposure of the steel base out of the plate layer and is also free fromcracks, pin holes, overplated spots and peeling of the plate layer. Thewelded tube of the invention therefore has excellent corrosionresistance and good workability for working the terminals. Theadvantages of the present invention are thus noticeable.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A method of producing a welded tube with anexcellent corrosion-resistant inner surface, in which a first platelayer of one metal material selected from Sn, Sn--Zn, Sn--Ni, Ni--P andNi--B is formed on at least one surface of a steel strip, then a secondplate layer of one metal material selected from Ni, Co and alloys basedon the metals is formed over the first plate layer, and the thus platedsteel strip is formed into a tube by welding with the plated surfacebeing inside and then heat-treated.
 2. The method of producing a weldedtube with an excellent corrosion-resistant inner surface as claimed inclaim 1, in which the thickness of the first plate layer is from 0.5 to10 μm.
 3. The method of producing a welded tube with an excellentcorrosion-resistant inner surface as claimed in claim 1, in which thethickness of the second plate layer is from 0.5 to 10 μm.
 4. The methodof producing a welded tube with an excellent corrosion-resistant innersurface as claimed in claim 1, in which the total thickness of the firstplate layer and the second plate layer is from 1 to 15 μm.
 5. The methodof producing a welded tube with an excellent corrosion-resistant innersurface as claimed in claim 1, in which the heat treatment is effectedat a temperature falling within the range of from the melting point ofthe single metal or alloy of forming the first plate layer to 1200° C.for a period of from 5 seconds to 15 minutes.
 6. The method of producinga welded tube with an excellent corrosion-resistant inner surface asclaimed in claim 5, in which the heat treatment is effected at atemperature of from 800° to 1200° C. for a period of from 10 seconds to5 minutes.